did you copy the key in the program folder of 3dgs ?

by the way, don't wory about the remaining days, if you have bought it, you can activate it, even if the demo's test days are depleted

get the C# wrapper:
for A7.85.4 and A8.30.4, Version 2.3.9
at http://acknexwrapper2.matthias-auer.net/ or visit the thread